1. Field of the Invention
This invention relates to a semiconductor pressure sensor and, more particularly, it relates to a highly sensitive semiconductor pressure sensor that sensitively detects signals generated under a static or differential pressure condition to accurately determine any static or differential pressure as well as a semiconductor pressure sensor that can be used under very high ambient pressure.
2. Description of the Related Art
Various pressure sensors are known, including semiconductor pressure sensors that are popularly used for differential pressure transducers.
A semiconductor pressure sensor comprises a thin diaphragm of a monocrystalline semiconductive material (e.g., silicon) having an excellent elasticity to accommodate any possible stress produced therein. By sensing the internal stress which is a function of the difference of the pressures applied to the both sides of the diaphragm, it generates and transmits an electric signal representing the difference of pressure.
FIGS. 1A and 1B of the accompanying drawings respectively illustrate a sectional view and a plan view of a conventional semiconductor pressure sensor.
Referring to FIG. 1A, the semiconductor pressure sensor 100 comprises a thin diaphragm 117 made of a monocrystalline silicon substrate 110 having n-type surfaces and formed with a cavity 115 on a side. Such a semiconductor pressure sensor is normally prepared by diffusing a p-type impurity substance on the surface opposite to the one having the cavity 115 to form differential pressure sensors (resistors of a strain gauge) 136A through 136C, forming thereafter a film of an oxide 161 on the semiconductor substrate 110 to cover said differential pressure sensors 136A through 136C and electrically connecting with wires the opposite ends of each of the differential pressure sensors 136A through 136C to respective electrode layers 164 by way of respective holes 162 cut through the oxide film 161. The semiconductor pressure sensor is secured to a hollow and tubular seat 120 and the electrode layer 164 is electrically connected to external terminals (not shown) by wires 163 constructed such as metal wires when used.
Referring now to FIG. 1B, while electrode layers 164 are shown only for the differential pressure sensors 136A through 136C located on the right half of the substrate, such electrode layers 164 are actually formed for all the differential pressure sensors 136A through 136C on the substrate 110.
When the diaphragm 117 of a semiconductor pressure sensor 100 having a configuration as described above is put under stress due to pressure applied thereto, of the differential pressure sensors 136A through 136C formed on the surface of the diaphragm 117, the resistance of only those differential pressure sensors having crystal axes disposed in a certain given direction is raised, whereas the resistance of all the other differential pressure sensors whose crystal axes are arranged in other predetermined directions decreases. Then, the output voltage of a Wheatstone bridge circuit (not shown) constituted by these differential pressure sensors 136A through 136C changes as a function of the change in resistance and a detection signal representing the difference of the output voltage before and after the change is transmitted from it.
A semiconductor pressure sensor 100 of the above described type can generate erroneous signals even when it is subjected to static pressure (pressure commonly applied to the both surfaces of the diaphragm), a phenomenon called a zero shift error.
In a solution that has been proposed for getting rid of zero shift errors, an additional pressure sensor device is used to detect the static pressure applied to it and compensate therewith the output of the semiconductor pressure sensor 100.
This solution, however, requires two sensor devices and, therefore, additional cost. It may be needless to say that a semiconductor pressure sensor that comprises only a single sensor device and still can generate an additional set of signals for correcting the first set of signals is, if realized, by far preferable.
Such a semiconductor pressure sensor is actually known and comprises sensors (static pressure sensors) for detecting strain that may be generated by static pressure. The sensors are arranged on the upper surface 110A of a silicon monocrystalline substrate 110 just as the differential pressure sensors 136A through 136C of a semiconductor pressure sensor as described above so that the outputs of said differential pressure sensors 136A through 136C may be corrected by the corresponding outputs of the static pressure sensors.
The static pressure sensors of such a semiconductor pressure sensor 100 are arranged on a single and same substrate commonly used for differential pressure sensors 136A through 136C to form a pattern of arrangement corresponding to that of the differential pressure sensors 136A through 136C in order for the static pressure sensors to be influenced by ambient temperature in a manner exactly same as the manner in which the differential pressure sensors are affected by ambient temperature. This means that the static pressure sensors are also affected by differential pressure induced strain and, therefore, the output signals of the semiconductor pressure sensor are not necessarily unequivocal and do not always show same response characteristics and the output signals of the static pressure sensors by turn need to be corrected by those of the respective differential pressure sensors, requiring very complicated operations for correction.
In other words, the above described method of correcting the signals of differential pressure sensors by those of static pressure sensors can give rise to large errors.
FIGS. 2A and 2B illustrate another conventional semiconductor pressure sensor which is used for a differential pressure transducer. The components of the semiconductor pressure sensor of FIGS. 2A and 2B that are similar to those of its counterpart of FIGS. 1A and 1B are indicated by same reference numerals and will not be described any further.
The semiconductor pressure sensor 100 of FIGS. 2A and 2B has a cylindrical cavity 115 cut from a surface thereof and is secured to a tubular seat 120 at the side of the cylindrical cavity 115. A pressure guide path 125 is formed in the tubular seat 120 to transmit the applied pressure to the cylindrical cavity 115.
The semiconductor pressure sensor 100 comprises a thin diaphragm section 117 located above the cylindrical cavity 115. The diaphragm section 117 is provided with a pair of radially aligned strain sensors 135A, 135B and another pair of strain sensors 130A, 130B disposed on a radial line perpendicular to the line connecting the sensors 135A, 135B and disposed perpendicular to the line connecting them. The sensors are arranged on the surface of the diaphragm 117 opposite to the one where the cylindrical cavity 115 is formed. The sensors 130A, 130B, 135A, 135B are differential pressure sensors for sensing the difference between the pressures applied to the opposite surfaces of the thin diaphragm 117. The differential pressure sensors and static pressure sensors are in fact stress sensors having certain piezoelectric resistance values that vary as a function of the change in the internal stress of the semiconductor chip. The differential pressure sensors are so wired as to form a bridge circuit that transmits a signal representing the difference between the pressures applied to the opposite surfaces of the thin diaphragm section 117.
Pairs of static pressure sensors 140A, 140B, 145A, 145B are arranged on the substrate where the differential pressure sensors are located in order to detect any strain that may be present under a static pressure condition and show a pattern of arrangement corresponding to that of the differential pressure sensors. Here, a static pressure condition means a condition where the two opposite surfaces of the diaphragm are subjected to a same and equal pressure and, therefore, no differential pressure is existent. The static pressure sensors and the differential pressure sensors are in fact stress sensors that are mutually connected to form a bridge circuit.
A semiconductor pressure sensor having a configuration as described above generates and transmits a differential pressure detection signal whenever the differential pressure sensors detect any strain produced in the thin diaphragm section 117. However, the differential pressure sensors sense strain which is produced in the thin diaphragm section 117 under a static pressure condition. In order to avoid this problem, the differential pressure detection signal is corrected by a signal obtained from the static pressure sensors that sense strain produced in the thin diaphragm section only under a static pressure condition.
A semiconductor pressure sensor as described above can be used for measurement of not only differential pressure but also pressure of other types. A static pressure condition can be obtained when such a semiconductor pressure sensor is placed in an atmosphere whose pressure is to be measured. Then, the pressure can be determined from the signals obtained from the static pressure sensors 140A, 140B, 145A, 145B.
A pressure sensor of the above described type, however, does not necessarily provide unequivocal signals for a stress field that appears in the diaphragm of the pressure sensor. Besides, the thin diaphragm can be damaged when large pressure that exceeds the rated pressure is applied thereto.
As described above, any conventional semiconductor pressure sensors can hardly provide unequivocal and reproducible linear signals and can be damaged when it is subjected to excessively large pressure.
Laid Open Japanese Patent Application No. 54-51489 and U.S. Pat. No. 4,530,244 discloses technologies related to semiconductor pressure sensors of the above described types.